This invention relates to mechanical end face seals. More particularly, it relates to an arrangement for noise attenuation in such seals.
Mechanical end face seals are employed in a wide variety of industrial applications such as liquid pumps, mixing vessels and the like to provide a fluid tight seal between a housing and a rotatable shaft. Such seals include an annular stationary ring associated with the housing and an annular rotatable ring associated with the shaft which includes sealing faces in a relatively rotating sealing relation along a seal interface. In pumps, the seal interface is lubricated by the fluid to be sealed, or a separate barrier fluid introduced into the seal chamber.
In mixers and agitator applications, the seals are employed to contain the product being mixed, usually by a vertically rotating agitator within a vessel. These seals, typically mounted near the top of the vessel are dry running, in that they operate in the vapor of the product mix or a gaseous atmosphere such as an air or nitrogen barrier fluid.
Dry running contacting face seals are commonly used to seal rotating shafts on mixers and agitators found throughout the chemical and pharmaceutical processing industries. A continuing problem experienced with dry running contacting seal faces, is that over time they develop an audible high pitched squeal or intermittent squeak. Many chemical and especially pharmaceutical production lines are located within enclosed buildings where the seal face noise creates an unpleasant and deleterious working environment.
Cause of the squealing phenomenon is generally thought to be related to harmonic vibration generated as a result of the rubbing characteristics of the seal face material properties, seal ring structure and equipment operational influences. The most common experience is on top entering mixers where the seal is mounted on the shaft that extends vertically down through the vessel flange opening. The seal head generally rotates with the shaft and the stationary face or mating ring is mounted against the vessel flange. The seal runs dry as the vessel normally has a vapor zone at the top of the product chamber. Most common seal designs for such applications are O-ring pusher seals with multiple springs and axial drive pins or lugs.
Seal face material selection is quite narrow being limited to those materials uniquely capable of providing suitable wear and sealing performance under various dry running application conditions. Select specialty carbon grade seal faces are typically utilized running against either a tungsten or silicon carbide counter face.
Several attempts to attenuate the squealing have been made that have not been successful. These fall into basic categories; materials and damping. Material changes are very limited due to the primary dry running function that must be maintained. To date, acceptable dry running face materials all still appear to exhibit squealing in operation.
Vibration damping methods have been applied to both the mating ring and the primary ring. One unsuccessful attempt at eliminating the noise was to place a shock absorbent pad behind the mating ring in order to reduce vibration. There have also been attempts to deaden the carbon primary ring by pressing a solid stainless steel ring on the outer diameter. This also has been proven to be ineffective in preventing faces from squealing.
Previously, noise attenuation efforts were limited to additions to the seal face that added mass, or changes to the seal face material to add mass or change the rate of wear or damping. Also, some effort had been made in the past to control the atmosphere surrounding the seal i.e., humidifying barrier gas or changing barrier pressure.
This invention seeks to address the distortion effects that stresses have on the seal face and how those stresses affect rubbing characteristics that create noise generation.
It is believed that as the relatively rotating seal faces run in, which occurs quite quickly on dry running contacting faces, the surfaces self-polish, creating an interface environment that is devoid of lubrication and is the mechanism that sets up vibration harmonics that result in the audible squealing. It is believed that by imparting some waviness to the seal face, it will prevent the high self-polishing action and permit an adequate level of lubrication to exist between the seal faces sufficient to prevent the onset of vibration induced squealing.
The present invention embodies a metal ring pressed on an outer diameter of the carbon ring near the sealing face that has intermittent sections on the ring inside diameter contacting the carbon with an interference fit at a number of designated asymmetrically located positions. This ring design imparts stresses on the carbon which, in turn, result in slight deformations on the seal face. The stress ring exerts circumferential pressure on the primary seal face which is unequal, and thus is ever changing depending upon the rotational speed of the shaft and axial load exerted on the seal. Thermal changes at the seal faces also affect the changes in stress. The changing state of stress on the primary ring serves to interrupt harmonic vibration at the seal faces and thus results in quiet dry running operation.
The arrangement of the present invention imparts radial pressure at locations around the outer surface of one seal ring at unequal or asymmetrical locations. The asymmetrical placement serves to interrupt the natural harmonics of resonance of the primary ring. A stress band or compression ring surrounds an outer cylindrical surface of one of the seal rings near the radially directed annular seal face. The band includes a plurality of radially inwardly directed pads disposed asymmetrically about the ring circumference. These pads are in interference fitting contact with the outer cylindrical surface of the seal ring. It imparts radially inward compressive forces to the annular seal ring. The resultant stresses distort the generally planar annular seal face.
The radial forces imparted to the seal ring face create distortion along the generally planar seal face. It is contemplated that such distortion or waviness may also be useful in liquid seals. It is contemplated that such a relationship between the seal faces in liquid pump applications may improve lubrication capabilities by the barrier liquid or process fluid leading to enhanced seal durability. This invention therefore also has application to liquid seals.